When a crossroads, plot point, discovery or choice happens in a GMless game someone (either first person to pipe up or someone chosen at random) suggests an outcome or result. The rest of the table then agrees on the likelihood of the outcome expressed as a number at which or higher on a D12 the outcome seems likely. The chance should be arranged so that a high roll is more favorable to the players, and a low roll is less favorable. This number could be generated as an average of all the suggested values given at the table. A D12 is rolled.
If a '12' is rolled, the event is wildly true to an unexpected degree.
If a '1' is rolled, the exact opposite of the outcome occurs. What “the opposite” in each case should exactly be should be more or less obvious to all at the table. If not, more brainstorming is necessary to negotiate what “the opposite” is. All should be encouraged to add details.
If the roll falls underneath the likelihood roll number, but is not a '1' it means that the situation should “go another way” and a different and unrelated suggestion should be suggested as an outcome or result and the process starts again.
If the roll “goes another way” three times in a row then the crossroads, plot point, discovery or choice becomes benign and simply results in the simplest least dangerous and mundane result possible. For example, that noise outside is just from a curious skunk or the book bound in arcane death glyphs is just a fancy cookbook.